Add Commercegate gateway

http://www.commercegate.com/

Close #1027.
Wirecard: Fix "amex" references

Close #1263.
HPS: Do not pass empty elements

In particular, do not attempt to send any CardHolder element unless
there is a first name or last name available at a minimum.

Also do not send an empty CVV element.

Closes #1269.
Paymill: Add more supported card types

See #1264.
Paymill: Add source

See #1264.
Paymill: Add description for preauthorizations

See #1264.
Paymill: Add remote tests using tokens

Closes #1264.
HPS: Add additional credential options

Close #1271.
Update 1stPayGateway.Net gateway

This replaces the now broken 1stPayGateway implementation with support
for their modern API and all five major transaction types.

Closes #1272.
Payflow: Add verbosity option

Closes #1275.